1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric receiver, piezoelectric sounder, piezoelectric speaker, and piezoelectric buzzer, and a manufacturing method thereof.
2. Description of the Related Art
In conventional electronic devices, home electronic appliances, portable telephones, and other such apparatuses, piezoelectric electroacoustic transducers have been widely used as a piezoelectric buzzer or piezoelectric receiver for providing an alarm sound or operating sound. A configuration of such a piezoelectric electroacoustic transducer generally is that a piezoelectric element is bonded on one surface of a metallic plate to define a unimorph-type diaphragm, the periphery of the metallic plate is supported in a case, and an opening of the case is closed with a cover.
However, the unimorph-type diaphragm has a drawback in that the displacement, i.e., sound pressure is small because a ceramic plate, which expands and contracts in the external diameter by voltage application, is bonded on a rigid metallic plate so as to flexually vibrate.
Then, a bimorph-type diaphragm having a deposited structure including a plurality of piezoelectric ceramic layers is disclosed in Japanese Unexamined Patent Application Publication No. 2001-95094. This diaphragm is configured such that two or three piezoelectric ceramic layers are deposited to form a deposited product having external electrodes disposed on the front/rear surfaces of the product and internal electrodes disposed between respective layers. All the ceramic layers are polarized in the same thickness direction, and by applying an alternating signal to between the external and internal electrodes, the deposited product is flexurally vibrated.
Such a diaphragm of the deposited structure has an advantage that a larger displacement, i.e., larger sound pressure, can be obtained in comparison with a unimorph-type diaphragm.
When manufacturing the diaphragm of the deposited structure described above, there is a problem in that the risk of a short circuit may be created between the internal electrode exposed on an end surface of the deposited product and the external electrode because of migration due to the very small thickness of each ceramic layer.
As an anti-migration measure, as shown in FIG. 1, there may be an electrode-forming method in which front/rear external electrodes 2 and 3 are exposed to at least one side of a ceramic layer 1 and trimmed parts 2a and 3a, from which the external electrodes 2 and 3 are cut out, are disposed on the other sides, while a trimmed part 4a of an internal electrode 4 is disposed on one side, at which the external electrodes 2 and 3 are exposed, and the internal electrode 4 is exposed at the remaining sides. In addition, the rear external electrode 3 is depicted as a projected figure in FIG. 1. From such an electrode configuration, on each side surface of the ceramic layer 1, the external electrodes 2 and 3 cannot come close to the internal electrode 4 in the thickness direction, thereby eliminating the migration.
In addition, referring to FIG. 1, on the respective three sides of the external electrodes 2 and 3, the trimmed parts 2a and 3a are formed while the trimmed part 4a is formed on the one side of the internal electrode 4. Conversely, even when forming the trimmed part on the three sides of the internal electrode 4 and the respective trimmed parts on the one side of the external electrodes 2 and 3, the same advantage can be obtained.
However, when polarization is performed on a deposited product having such an electrode configuration by applying a DC voltage, there is a problem that cracks in the ceramic layer 1 may be produced in the boundary between the internal electrode 4 and the trimmed part 4a because of the expansion difference of the ceramic layer 1 in between the internal electrode 4 and the trimmed part 4a, reducing the yield ratio. That is, the side of the ceramic layer 1 having the internal electrode 4 exposed on the side surface is prevented from expanding by the internal electrode 4, whereas the side of the ceramic layer 1 having the trimmed part 4a expands largely, so that the cracks are produced in the ceramic layer 1 by the expansion difference.